1. Field of the Invention
This invention relates to an improved compact oxygen sensor and, more specifically, this invention relates to a method of making a high-temperature solid-state potentiometric oxygen sensor with an internal reference sealed by plastic joining.
2. Background of the Invention
Oxygen sensors are required to monitor oxygen levels in combustion and industrial processes. Typical oxygen sensors require a continuous supply of reference oxygen (usually air). Such sensors are expensive inasmuch as complex plumbing is required to route the reference gas to the sensor. As a result of the need for such plumbing, the locations of the sensors are also restricted and usually relegated to more mild locations within a process stream. For example, typical oxygen sensors are usually placed inside the flue of a combustion system and not in the actual combustion chamber. Indeed, temperatures approaching 1600° C. are often reached in such combustion chambers.
There are two types of oxygen sensors: amperiometric and potentiometric. Amperiometric sensors require a voltage source to function.
Potentiometric sensors require an external gas source. Moreover, additional plumbing is required to route said external gas to an interior region of the sensor. Potentiometric sensors contain internal references which require glass seals, metal housings, and similar materials which are not robust and not suited to thermal cycling. As such, typical potentiometric sensors are restricted to low temperature operations.
Past efforts in fabricating autonomous sensors (i.e., not requiring external plumbing) have had only partial success. For example, U.S. Pat. No. 5,827,415 awarded to Gur et al. on Oct. 27, 1998, utilizes a solid electrolyte slab having a first side covered with a sensing electrode and a second side containing a metal-metal oxide reference electrode. The metal-metal oxide electrode is isolated from the environment using glass seals to keep out oxygen. However, glass seals limit the operational temperature and become brittle with temperature cycling.
U.S. Pat. No. 3,915,830 awarded to Isenberg on Oct. 28, 1975 discloses a reference medium encapsulated by solid electrolyte, the later of which is sputtered, plated or otherwise deposited onto the medium. A lead wire from the reference medium is routed to outside of the resulting construct and sealed with glass. As mentioned supra, glass tends to become brittle during temperature cycling and limits the operational temperature.
U.S. Pat. No. 6,440,028 awarded to Kim, et al. on Aug. 27, 2002 discloses a manual valve of a hydraulic pressure control system for automatic transmission. The invention requires an atmospheric reference and a voltage source. Numerous different materials are involved in its manufacture, several of which restrict thermal cycling applications.
U.S. Pat. No. 5,360,528 awarded to Oh, et al. on Nov. 1, 1994 discloses a wide range oxygen sensor. However, the applicable current is limited, many materials are required for manufacture, and there is restricted range of thermal cycling and temperature operation.
U.S. Pat. No. 5,543,025 awarded to Garzon, et al. on Aug. 6, 1996 and U.S. Pat. No. 5,695,624 awarded to Garzon, et al. on Dec. 9, 1997 both disclose a solid state oxygen sensor with a yttria-doped zirconia as an electrolyte. Both inventions require glass seals, which limit operational temperature and thermal cycling due to the glass' tendency to become brittle.
U.S. Pat. No. 4,502,939 awarded to Holfelder, et al. on Mar. 5, 1985 discloses an electrochemical oxygen sensor, particularly for analysis of combustion cases from internal combustion engines. It requires metal housing, limits operational temperature and limits thermal cycling.
Thin film sensors, as in most of the patents described supra, are no longer available because they are expensive to make and not robust.
A need exists in the art for a high-temperature oxygen sensor that is compact and inexpensive, and a method for producing such a sensor. The sensor should obviate the need for reference oxygen plumbing and therefore the need for oxygen gas as reference pO2. The sensor should also be robust in a myriad of environments, and particularly in high-temperature and fluctuating temperature situations such as effluent streams, industrial waste streams, combustion streams, process streams, NOx traps, and turbines. Such a robust nature would be the result of a fabrication process utilizing plastic deformation to seal or otherwise isolate internal reference material from the harsh environments.